1. Field of the Invention
The present invention relates to a high-frequency component that can be utilized in a plurality of mobile communications systems.
2. Description of the Related Art
A mobile communication apparatus is available that is capable of performing communication in a plurality of communications systems each having a different frequency band, such as 1800-MHz-band GSM1800 (DCS), 1900-MHz-band GSM1900 (PCS), 850-MHz-band GSM850, and 900-MHz-band GSM900 (EGSM).
Such a mobile communication apparatus utilizes a front-end unit that performs signal splitting and merging for supporting multiple bands, such as quadruple bands compatible with four communications systems, triple bands compatible with three communications systems, and dual bands compatible with two communications systems (see, for example, Japanese Unexamined Patent Application Publication No. 2004-94410).
In general, the front-end unit of such a mobile communication apparatus supporting multiple bands is configured as a module that includes a diplexer connected to an antenna port and a plurality of switching circuits connected to the diplexer in stages subsequent thereto.
For instance, in the front-end unit of a mobile communication apparatus supporting dual bands, a diplexer performs splitting/merging for a transmission/reception signal in the low-frequency band used by EGSM systems (GSM900 and GSM850 systems) etc. and a transmission/reception signal in the high-frequency band used by systems such as DCS or PCS. In a low-frequency-band circuit in a stage subsequent to the diplexer, an EGSM switching circuit performs switching between an EGSM transmission signal and an EGSM reception signal. Similarly, in a high-frequency-band circuit in a stage subsequent to the diplexer, a switching circuit performs switching between a DCS (PCS) transmission signal and a DCS (PCS) reception signal.
In the front-end unit of a mobile communication apparatus that supports triple bands or quadruple bands, for example, the above-described switching circuit is further provided with another switching circuit connected thereto in a subsequent stage for performing, for example, switching between a GSM850 reception signal and a GSM900 reception signal, or switching between a DCS reception signal and a PCS reception signal.
The reception path corresponding to each communications system in such a front-end unit may have an unbalanced-input/balanced-output SAW filter, which passes only a signal-reception band, while removing signals in undesired frequency bands, and amplifies a received signal. This SAW filter prevents a failure of a circuit on the reception path side caused by a signal that leaks from a transmission path to the reception path.
A diplexer, in general, has a circuit configuration in which a high-frequency-band filter and a low-frequency-band filter are connected in parallel to the antenna port.
The high-frequency-band filter may be configured to include a plurality of capacitors connected serially to the antenna port and a series resonant circuit, one end of which is connected to a connection node among the capacitors and the other end of which is grounded. These configuration elements are made to have impedance settings so as to attenuate a transmission/reception signal in the low-frequency band and to pass a transmission/reception signal in the high-frequency band.
To realize a steep attenuation pole, in the cutoff characteristic, for preventing a transmission/reception signal in the low-frequency band from entering the high-frequency-band side, the impedances of the configuration elements of the series resonant circuit of the high-frequency-band filter are set so that the resonant circuit has a resonance frequency that corresponds to the trap frequency of a transmission/reception signal in the low-frequency band (for example, a resonance frequency equal to the specification-defined center frequency of a communications system for the low-frequency band).
The low-frequency-band filter is configured to include a parallel resonant circuit having a line connected to the antenna port and a capacitor connected in parallel to this line and to include a series resonant circuit having a line and a capacitor serially connected to this line. These configuration elements are made to have respective impedance settings so as to attenuate a transmission/reception signal in the high-frequency band and to pass a transmission/reception signal in the low-frequency band.
To realize a steep attenuation pole, in the transmission characteristic, for preventing a transmission/reception signal in the high-frequency band from entering the low-frequency-band side, the respective impedances of the configuration elements of the parallel resonant circuit of the low-frequency-band filter are set so that the resonant circuit has a resonance frequency that corresponds to the trap frequency of a transmission/reception signal in the high-frequency band (for example, a resonance frequency equal to the specification-defined center frequency of a communications system).
When a SAW filter is provided on a reception path of each communications system, impedance matching is not realized in the connection portion of the SAW filter because the impedance of the SAW filter for frequencies outside of the passband is not 50Ω. Hence, there is a problem in that it is necessary, for impedance matching, to provide a phase adjustment circuit in the front-end unit or to provide a multi-stage filter in the diplexer, causing a complicated circuit configuration and a resultant increase in the number of components and size of the module.
If a SAW filter is provided on a reception path of a communications system of the high-frequency band or the low-frequency band without performing impedance matching, the transmission characteristic of the frequency band of one communications system and the cutoff characteristic of the frequency band of the other communications system will be degraded.